POLθ prevents MRE11-NBS1-CtIP-dependent fork breakage in the absence of BRCA2/RAD51 by filling lagging-strand gaps.

POLθ promotes repair of DNA double-strand breaks (DSBs) resulting from collapsed forks in homologous recombination (HR) defective tumors. Inactivation of POLθ results in synthetic lethality with the loss of HR genes BRCA1/2, which induces under-replicated DNA accumulation. However, it is unclear whether POLθ-dependent DNA replication prevents HR-deficiency-associated lethality. Here, we isolated Xenopus laevis POLθ and showed that it processes stalled Okazaki fragments, directly visualized by electron microscopy, thereby suppressing ssDNA gaps accumulating on lagging strands in the absence of RAD51 and preventing fork reversal. Inhibition of POLθ DNA polymerase activity leaves fork gaps unprotected, enabling their cleavage by the MRE11-NBS1-CtIP endonuclease, which produces broken forks with asymmetric single-ended DSBs, hampering BRCA2-defective cell survival. These results reveal a POLθ-dependent genome protection function preventing stalled forks rupture and highlight possible resistance mechanisms to POLθ inhibitors.

Epigenetic regulation of replication origin assembly: A role for histone H1 and chromatin remodeling factors.

During early embryonic development in several metazoans, accurate DNA replication is ensured by high number of replication origins. This guarantees rapid genome duplication coordinated with fast cell divisions. In Xenopus laevis embryos this program switches to one with a lower number of origins at a developmental stage known as mid-blastula transition (MBT) when cell cycle length increases and gene transcription starts. Consistent with this regulation, somatic nuclei replicate poorly when transferred to eggs, suggesting the existence of an epigenetic memory suppressing replication assembly origins at all available sites. Recently, it was shown that histone H1 imposes a non-permissive chromatin configuration preventing replication origin assembly on somatic nuclei. This somatic state can be erased by SSRP1, a subunit of the FACT complex. Here, we further develop the hypothesis that this novel form of epigenetic memory might impact on different areas of vertebrate biology going from nuclear reprogramming to cancer development.

SSRP1-mediated histone H1 eviction promotes replication origin assembly and accelerated development.

In several metazoans, the number of active replication origins in embryonic nuclei is higher than in somatic ones, ensuring rapid genome duplication during synchronous embryonic cell divisions. High replication origin density can be restored by somatic nuclear reprogramming. However, mechanisms underlying high replication origin density formation coupled to rapid cell cycles are poorly understood. Here, using Xenopus laevis, we show that SSRP1 stimulates replication origin assembly on somatic chromatin by promoting eviction of histone H1 through its N-terminal domain. Histone H1 removal derepresses ORC and MCM chromatin binding, allowing efficient replication origin assembly. SSRP1 protein decays at mid-blastula transition (MBT) when asynchronous somatic cell cycles start. Increasing levels of SSRP1 delay MBT and, surprisingly, accelerate post-MBT cell cycle speed and embryo development. These findings identify a major epigenetic mechanism regulating DNA replication and directly linking replication origin assembly, cell cycle duration and embryo development in vertebrates.

Xenopus egg extract to study regulation of genome-wide and locus-specific DNA replication.

Faithful DNA replication, coupled with accurate repair of DNA damage, is essential to maintain genome stability and relies on different DNA metabolism genes. Many of these genes are involved in the assembly of replication origins, in the coordination of DNA repair to protect replication forks progression in the presence of DNA damage and in the replication of repetitive chromatin regions. Some DNA metabolism genes are essential in higher eukaryotes, suggesting the existence of specialized mechanisms of repair and replication in organisms with complex genomes. The impact on cell survival of many of these genes has so far precluded in depth molecular analysis of their function. The cell-free Xenopus laevis egg extract represents an ideal system to overcome survival issues and to facilitate the biochemical study of replication-associated functions of essential proteins in vertebrate organisms. Here, we will discuss how Xenopus egg extracts have been used to study cellular and molecular processes, such as DNA replication and DNA repair. In particular, we will focus on innovative imaging and proteomic-based experimental approaches to characterize the molecular function of a number of essential DNA metabolism factors involved in the duplication of complex vertebrate genomes.

Replication, checkpoint suppression and structure of centromeric DNA.

Human centromeres contain large amounts of repetitive DNA sequences known as α satellite DNA, which can be difficult to replicate and whose functional role is unclear. Recently, we have characterized protein composition, structural organization and checkpoint response to stalled replication forks of centromeric chromatin reconstituted in Xenopus laevis egg extract. We showed that centromeric DNA has high affinity for SMC2-4 subunits of condensins and for CENP-A, it is enriched for DNA repair factors and suppresses the ATR checkpoint to ensure its efficient replication. We also showed that centromeric chromatin forms condensins enriched and topologically constrained DNA loops, which likely contribute to the overall structure of the centromere. These findings have important implications on how chromosomes are organized and genome stability is maintained in mammalian cells.

H ferritin gene silencing in a human metastatic melanoma cell line: a proteomic analysis.

Ferritin, the major intracellular iron-storage protein, is made of 24 subunits of two types, H and L. Besides regulating intracellular iron homeostasis, it has been found that ferritin, in particular the H subunit (FHC), is involved in different biological events such as cell differentiation and pathologic states (i.e., neurodegeneration and cancer). This study is aimed at investigating the whole-cell proteome of FHC-expressing and sh-RNA-silenced human metastatic melanoma cells (MM07(m)) in the attempt to identify and classify the highest number of proteins directly or indirectly controlled by the FHC. We identified about 200 differentially expressed proteins and classified them in clusters on the basis of their functions, as proteins involved in metabolic processes, cell adhesion, migration, and proliferation processes. Some of them have captured our attention because of their involvement in metabolic pathways related to tumor progression and metastasis. In vitro assays confirmed that the FHC-silenced MM07(m) cells are characterized by a decreased growth activity, a reduced invasiveness, and a reduced cell adhesion capability. Moreover, nude mice (CD1 nu/nu), subcutaneously injected with FHC-silenced MM07(m) cells, showed a remarkable 4-fold reduction of their tumor growth capacity compared to those who received the FHC-unsilenced MM07(m) counterpart. In conclusion, these data indicate that gene silencing technology, coupled to proteomic analysis, is a powerful tool for a better understanding of H ferritin signaling pathways and lend support to the hypothesis that specific targeting of this gene might be an attractive and potentially effective strategy for the management of metastatic melanoma.

BRCA1 is required for hMLH1 stabilization following doxorubicin-induced DNA damage.

Human DNA mismatch repair (MMR) is involved in the removal of DNA base mismatches that arise either during DNA replication or are caused by DNA damage. In this study, we show that the activation of the MMR component hMLH1 in response to doxorubicin (DOX) treatment requires the presence of BRCA1 and that this phenomenon is mediated by an ATM/ATR dependent phosphorylation of the hMLH1 Ser-406 residue. BRCA1 is an oncosuppressor protein with a central role in the DNA damage response and it is a critical component of the ATM/ATR mediated checkpoint signaling. Starting from a previous finding in which we demonstrated that hMLH1 is able to bind to BRCA1, in this study we asked whether BRCA1 might be the bridge for ATM/ATR dependent phosphorylation of the hMLH1 molecular partner. We found that: (i) the negative modulation of BRCA1 expression is able to produce a remarkable reversal of hMLH1 stabilization, (ii) BRCA1 is required for post-translational modification produced by DOX treatment on hMLH1 which is, in turn, attributed to the ATM/ATR activity, (iii) the serine 406 phosphorylatable residue is critical for hMLH1 activation by ATM/ATR via BRCA1. Taken together, our data lend support to the hypothesis suggesting an important role of this oncosuppressor as a scaffold or bridging protein in DNA-damage response signaling via downstream phosphorylation of the ATM/ATR substrate hMLH1.